Actually it is possbile, like trickykid said, some company are expertized in such operations.
For that you have to open the drive.
The technology is simple. When you write zero, it's not 0, it's maybe 0.002 if there was a 1 before and 0.00001 if it was zero before.
Each bit has like a "memory" of the precedent values it had
I hope I was clear